This invention relates to a transformer which can handle a high-frequency large current, which may be used, for example, with an inverter.
An example of prior art transformer handling a high-frequency large current is shown in FIGS. 1A and 1B. In FIG. 1A, primary and secondary coils of ribbon-shaped conductors are wound on a bobbin 41. The primary coil has winding start terminal 42 and a winding end terminal 43. The secondary coil has a winding start terminal 44 and a winding end terminal 45. These components form a coil unit 47. E-shaped core halves 48 and 49 are inserted into a center hole of the bobbin 41 from opposite sides of the hole to such an extent that the front ends of the core halves 48 and 49 abut against each other. This complete a transformer shown in FIG. 1B.
As is seen from FIG. 1B, the thickness H of the transformer is the sum of the thickness T of the core formed by the core halves 48 and 49, the thickness U of the coils on one side and the thickness V of the coils on the opposite side of the bobbin 41. Coils of transformers handling a large current, however, have an increased cross-sectional area, resulting in increased coil thicknesses U and V, which leads to increase of the overall thickness H of the transformer. In some cases, a heat sensing device, e.g. a thermistor, is disposed in intimate contact with the coils to avoid burnout of the coils. This causes a gap to be produced between coil layers, resulting in further increase of the coil thicknesses U and V.
Another example is shown in FIG. 2. The example shown in FIG. 2 is a transformer disclosed in U.S. Pat. No. 5,010,314, which is issued to A. Estrov on Apr. 23, 1991, entitled xe2x80x9cLOW-PROFILE PLANAR TRANSFORMER FOR USE IN OFF-LINE SWITCHING POWER SUPPLIESxe2x80x9d.
The transformer of Estrov uses planar conductors for coil windings to reduce the thickness of the coils. The transformer includes a printed circuit board 51 having a center window 52. Coil conductors 53 and 54 formed in loop are disposed on opposite major surfaces of the board 51. The conductors 53 and 54 are connected in series by soldering them through a through-hole 55.
The printed circuit board 51 has a tab 56 on which a winding start terminal 57 and a winding end terminal 58 are disposed. Disposed over the opposite major surfaces of the printed circuit board 51 are insulating sheets 61 and 62 having respective windows 59 and 60 and having the same shape and size as the printed circuit board 51 excluding the tab 56. In this manner, a stack 63 is formed.
A plurality of similar stacks 63 are prepared and stacked on the first stack to thereby form a coil unit 64. The winding start terminal 57 of one board 51 and the winding end terminal 58 of adjacent board 51 in the coil unit 64 are soldered together, whereby primary and secondary coils having desired numbers of conductor turns are formed.
Bobbins 67 and 68 each in the form of a short rectangular tube having flanges 65 and 66, respectively, are inserted into the window of the coil unit 64 from opposite sides of the unit 64. Then, E-shaped high-frequency core members 69 and 70 are inserted into the window to thereby complete the transformer.
The dimensions of the windows 52, 59 and 60 in the printed circuit board 51 and the respective ones of the insulating sheets 61 and 62 are equal to the outer dimensions of the rectangular tubular bobbins 67 and 68. The distance between the flanges 65 and 66 with the front end surfaces of the bobbins 67 and 68 abutting against each other is equal to the height of the coil unit 64. The shapes and sizes of the center leg of the core members 69 and 70 are conformal to the windows in the bobbins 67 and 68.
The current-carrying capacity in the transformer shown in FIG. 2 depends on the cross-sectional area of the conductors formed on the printed circuit board 51. Usually, the maximum thickness of a conductor realizable by the printed circuit board technology is 0.1 mm, and the manufacturing cost is proportional to the conductor thickness. With the conductor thickness of 0.1 mm or so, the board tends to warp or deform during the formation of the conductors, and, therefore, the thickness of the board itself cannot be less than 1.0 mm. When conductors 0.1 mm in thickness are formed on the opposite major surfaces of the board having a thickness of 1.0 mm, the ratio of the cross-sectional areas of the conductors to the cross-sectional area of the coil is 20% or less.
Even when deformation or warpage of an individual board produced during the formation of the conductors is small, the coil unit 64 formed of a stack of a plurality of such boards may swell due to warpage of the individual boards, and, therefore, the unit 64 cannot be properly placed between the flanges 65 and 66 of the bobbins 67 and 68. Also, if there are gaps between adjacent boards, vibrations and noise tend to be generated when current is supplied to the transformer. Also, such warpage will decrease reliability of soldered connections between conductors when a large current is supplied. For these reasons, the transformer shown in FIG. 2 has a limit in practical use. It can be used only with the primary input of 200 V and 2 A or so.
Therefore, an object of the present invention is to provide a thin, high-frequency transformer which can handle a large current.
A transformer according to an embodiment includes a plurality of planar coil members, each of which coil members is formed of a metal sheet. The planar coil member has a window in its center portion. A slit extends outward from the center window. First and second terminals are disposed on the sheet at locations on opposite sides of the slit.
A higher-voltage coil is formed by stacking a plurality of such coil members with an insulating sheet disposed between adjacent coil members. Instead, coil members each having an insulating sheet bonded to its one or both surfaces may be used. The first terminal of one coil member is connected to the second terminal of the adjacent coil member so that the coil members in the stack are connected in series.
A lower-voltage coil is formed of one or more coil members. The number of the coil members to be used is determined in accordance with a desired number of turns and desired current-carrying capacity. Specifically, for one turn of the lower-voltage coil, one planar coil member is used if it can provide a sufficient current-carrying capacity. If, on the other hand, the current-carrying capacity provided by one coil member is insufficient, a plurality of coil members connected in parallel are used as a coil member assembly for one turn. Further, if a plurality of turns are desired, a plurality of coil members or coil assemblies are stacked with an insulating sheet disposed between adjacent coil member or coil member assemblies like the higher-voltage coil. As in the high-voltage coil, coil members or coil member assemblies each having an insulating sheet bonded to its one or both surfaces can be used, without disposing an insulating sheet between adjacent coil members or coil assemblies.
The higher-voltage coil and the lower-voltage coils are stacked into a tubular coil unit with a window in its center portion. The coil unit is combined with a core having a portion extending through the window in the coil unit.
The planar coil members can be joined together by screwing, riveting, welding or brazing. When riveting is employed, coupling between terminals is more or less unreliable, causing increase of electrical resistance, but the resistance exhibited at the riveted portions can be reduced by applying solder over the riveted portions.
The core is suitably in the form of an 8-shaped frame including two outer legs spaced from a center leg with a window disposed between the center leg and each outer leg. The coil unit is placed around the center leg, with the coil members extending through the windows in the core. The width of each insulating sheet is substantially equal to the distance between the two outer legs, and the shape and size of the window in each insulating sheet are substantially same as those of the cross-section of the center leg. It is desirable that the width of the planar coil members is smaller than that of the insulating sheets, and that the width and length of the window in the planar coil members are larger than the width and length of the window in the insulating sheets, respectively, so that the planar coils can be prevented from contacting the core.
Instead of dimensioning the planar coil members and the insulating sheets in the manner as described above, the stack of the planar coils and insulating sheets may be surrounded by an insulating frame. The frame is provided with an projection on its inward facing surface, which protrusion is brought into engagement with a recess formed at a corresponding location in the outer periphery of the stack of planar coil members and insulating sheets. This arrangement enables the positioning of the planar coil members with respect to the insulating sheets and, at the same time, can prevent the planar coils from contacting the inner surface of the outer legs of the core.
An outwardly extending tab may be formed on one or more of planar coil members, with a heat sensing element mounted thereon to measure the temperature of the planar coils. With this arrangement, increase of the thickness of the coils due to the mounting of a heat sensing element can be avoided.